The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its advantages to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an express or implied admission that such art is widely known or forms part of common general knowledge in the field.
A grinding mill has a generally cylindrical mill chamber called a mill body or “shell” and two journal shafts, the journals being mounted upon supports for rotation. Large scale ball mills, typically more than 24′ in diameter and 40′ in length, are generally constructed by dividing the mill shell into individual components called “shell sections”, whereby the shell sections have “joints” between adjoining sections for facilitating later assembly on-site. The division of the mill shell into modular shell sections assists in the manufacture and transportation of the mill to the plant site. Depending on the size of the mill shell, there may be numerous splits in the lengthwise direction, each lengthwise split differentiating one shell “can” from the next shell “can”. Each shell can comprise one-half, one-third or one quarter of the mill shell diameter. Each shell section has at least one connecting element in the form of a flange that extends along at least one edge of each shell section. The flanges can either be longitudinal flanges or circumferential flanges, the former extending parallel to the longitudinal axis of the mill shell when assembled while the latter extend around the circumference of the mill shell. The mill shell is assembled by aligning the respective flanges of adjacent shell section and bolting them together to form what is generally known as a “split flange”.
A disadvantage of these split flanges is that there is frequently a significant amount of stress in the vicinity of the shell flange connection where a split flange meets another split flange. For example, a longitudinal split flange joining two shell sections is then connected to circumferential flange on a third shell section. This creates a “hard spot” which concentrates the stresses in the flanges in one area. As the longitudinal flange is typically quite stiff and thick, it causes a large load at the hard spot due to a sudden change in thickness of the joint where the longitudinal flange meets the circumferential flange, thus creating the concentrated area of high stress. The presence of hard spots adversely affects joint integrity, may cause cracking in the mill shell and eventually results in long term mill failure. High stress areas may also be created by other non-axisymmetric geometrical structural elements or discontinuities on the shell section, such as steps in thickness or man-holes which cause an increase in stress. A high stress can also form where the mill head is also split. This hard spot problem is exacerbated in large scale grinding mills discussed above, including ball mills, as the relative difference in stiffness caused by split flanges is greater due to a generally less stiff structure with increasing diameter. A greater length also reduces the global mill stiffness together with increasing the bending moment.
To address this issue, some mill manufacturers increase the mass of the shell section to reduce the impact of the stress concentrations resulting from hard spots arising from split flanges and other non-axisymmetric structural elements. Other mill manufacturers ignore the impact altogether and design as if there were no stress raisers. The former path has obvious disadvantages in increasing the mass of the mill and hence its manufacturing cost, but is technically superior to the latter path. The latter path results in a large mill with a lighter mass, but has stresses of an unknown magnitude in one or more critical locations, thus running the risk of stresses exceeding the allowable level and the abovementioned problems of mill shell cracking and long term mill failure. As mills of the size discussed have only been put into service in relatively recent years, these risks are not evident as yet and may only surface in the long term.
It is an object of the present invention to overcome or substantially ameliorate one or more of the disadvantages of prior art, or at least to provide a useful alternative.
It is an object of the invention in at least one preferred form to provide a stress distribution element for a mill shell section that minimises the effect of high stresses at concentrated areas or hard spots where longitudinal and/or circumferential flanges of adjacent mill shell sections meet, and a method for distributing stress from a joint using the stress distribution element.